Problem: A circle with area $4\pi$ has a sector with a $\dfrac{9}{5}\pi$ radian central angle. What is the area of the sector? ${4\pi}$ $\color{#9D38BD}{\dfrac{9}{5}\pi}$ ${\dfrac{18}{5}\pi}$
Answer: The ratio between the sector's central angle $\theta$ and $2 \pi$ radians is equal to the ratio between the sector's area, $A_s$ , and the whole circle's area, $A_c$ $\dfrac{\theta}{2 \pi} = \dfrac{A_s}{A_c}$ $\dfrac{9}{5}\pi \div 2 \pi = \dfrac{A_s}{4\pi}$ $\dfrac{9}{10} = \dfrac{A_s}{4\pi}$ $\dfrac{9}{10} \times 4\pi = A_s$ $\dfrac{18}{5}\pi = A_s$